Letter features Modi's photo juxtaposed against that of beheaded British aid worker David Haines.

The Bharatiya Janata Party (BJP) office in Kerala's capital has received a letter threatening harm to Prime Minister Narendra Modi, who is slated to visit Thiruvananthapuram next month.

The letter features Modi's photo juxtaposed against that of British aid worker David Haines, who was beheaded by the Islamic State, The Hindu has reported.

The letter is believed to have been sent by the Social Democratic Party of India (SDPI), a hardline Islamist political outfit.

In a separate incident that points to the growing support for Islamic State in Kerala, a pro-ISIS poster was seen in Thiruvananthapuram on Tuesday.

Days after the Kerala government was warned that the state was turning into a recruiting hub for ISIS, a poster supporting the terror outfit appeared near the Thampanoor police station in the capital according to a report by Marunadan Malayali, one of the largest online local language news portals in the state.

The police immediately removed the poster and are investigating the case as a serious issue.

The poster reportedly carried a message justifying the terror group's activities in Iraq and Syria.

A special team has been formed to find the people behind the poster and the police are checking CCTV footages from the area. The police will also strengthen night patrolling in the area.

The central home ministry had earlier sent out a report that ISIS was making inroads in Kerala.

Earlier, a central intelligence agency had informed the state that a resident of Palakkad was in touch with the group.